CVE-2024-56328 in Discourse
Summary
by MITRE • 02/04/2025
Discourse is an open source platform for community discussion. An attacker can execute arbitrary JavaScript on users' browsers by posting a maliciously crafted onebox url. This issue only affects sites with CSP disabled. This problem has been patched in the latest version of Discourse. Users are advised to upgrade. Users unable to upgrade should enable CSP, disable inline Oneboxes globally, or allow specific domains for Oneboxing.
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Analysis
by VulDB Data Team • 09/26/2025
CVE-2024-56328 represents a cross-site scripting vulnerability within the Discourse community discussion platform that allows attackers to execute arbitrary JavaScript code in users' browsers through crafted onebox URLs. This vulnerability specifically targets the platform's onebox functionality which automatically generates embedded content previews when users paste URLs into posts. The flaw exists because Discourse fails to properly sanitize URL inputs before rendering them in onebox previews, creating an avenue for malicious code injection that can persistently affect all users who view the compromised content.
The technical implementation of this vulnerability stems from the platform's insufficient input validation and output encoding mechanisms within its onebox processing system. When users post content containing maliciously crafted URLs, the Discourse platform processes these inputs without adequate sanitization, allowing attackers to inject JavaScript payloads that execute in the context of other users' browsers. This represents a classic cross-site scripting flaw that aligns with CWE-79 - Improper Neutralization of Input During Web Page Generation into a Web Browser, which specifically addresses the failure to properly encode or escape user-controllable data before including it in web page output.
The operational impact of this vulnerability extends beyond simple code execution, as it enables attackers to perform a range of malicious activities including session hijacking, credential theft, data exfiltration, and persistent browser-based attacks. Since the vulnerability only affects installations with Content Security Policy (CSP) disabled, it demonstrates the critical importance of proper security headers in mitigating client-side vulnerabilities. The attack vector is particularly concerning because it leverages the platform's legitimate onebox functionality to deliver malicious payloads, making detection more difficult and potentially allowing attackers to bypass traditional security monitoring systems.
Organizations using Discourse must understand that this vulnerability represents a significant risk to user privacy and platform integrity, particularly in environments where users may not be security-aware or where malicious actors could exploit the vulnerability to compromise user accounts. The recommended mitigations include immediate upgrading to patched versions of Discourse, which addresses the core sanitization issues in the onebox processing pipeline. When immediate upgrades are not feasible, administrators should implement proper CSP headers to prevent script execution, disable inline oneboxes globally to eliminate the attack surface, or establish strict allowlists for domains permitted in onebox functionality. These remediation strategies align with ATT&CK technique T1566.001 - Phishing: Spearphishing Attachment, as they address the initial compromise vector through input validation and output sanitization measures that prevent malicious code from executing in user browsers. The vulnerability underscores the critical need for proper input validation in web applications and demonstrates how seemingly benign features like content previews can become attack vectors when proper security controls are not implemented.